Koeman's team recorded a fourth league win in five games to move two points behind leaders Atletico Madrid ahead of Sunday's derby with rivals Real.
The Argentina playmaker then teed up substitute Ilaix Moriba for the 18-year-old to fire in a late second – his first Barcelona goal.
"We are trying to win every game, but that is the most difficult," Koeman told Barcelona's club media following the win in Pamplona.
"In the past few months we have a great run and are making good wins, important wins, which is always good, but the fighting spirit is really wonderful."
Koeman added: "It wasn't our best game, but we were OK and we deserved to win. I'm happy.
"The run of form is important. Let's hope we can stay on the right track.
"Any result tomorrow (in the Madrid derby) suits us."
Moriba's late strike saw him become Barcelona's fifth-youngest goalscorer and Koeman hopes to see more.
"Every time he comes into the team, he adds something," the Barca boss said.
"He is good on the ball and is a strong boy physically. He's pushing to be in the first team more often.
"We're really proud of being able to bring younger players through the ranks. We do it because they deserve to have opportunities.
"As a coach, I've always given opportunities to youngsters because they are a club's future."
Osasuna coach Jagoba Arrasate accepted his side had not made the most of their chances.
"I have the feeling that we made their goalkeeper work harder than ours did," Arrasate said on the club's official website. "But in the end, it's also about efficiency.
"We had a moment, at 0-1, when it looked like we could tie the game with crosses from the flanks, but it didn't last long and it wasn't possible.
"We have to pick ourselves up now because we have another game on Saturday."